Aggregation reaction of antigen-antibody combinations in microbiological diagnosis

2021-08-22 10:13 PM

Both living and dead microorganisms are capable of agglutination with antibodies. With live microorganisms, carry out the reaction on a slide. This reaction is commonly used to identify bacteria

Principles

An agglutination reaction is the fusion of a visible antigen with the corresponding antibody, forming agglutination particles that are visible to the naked eye.

Antigens can be bacteria, red blood cells, white blood cells, spermatozoa, etc. The agglutination reaction occurs only in the presence of electrolytes, most obvious, fastest at pH 7 to 7.2 and at temperature 37 0 C.

Direct agglutination reaction

Both living and dead microorganisms are capable of agglutination with antibodies. With live microorganisms, carry out the reaction on a slide. This reaction is commonly used to identify bacteria. In addition to bacteria, cells such as red blood cells, sperm... all have the ability to agglutinate with reciprocal antibodies.

In the case of antigens that are dead microorganisms, perform an in vitro reaction to determine the serum antibody titer in the diagnosis of disease, such as the Widal reaction in the diagnosis of typhoid.

Indirect agglutination reaction

Here antigens and antibodies only agglutinate in the presence of a third factor. The Coombs reaction is an example. The Rh- mother gives birth to an Rh+ (D antigen) baby. During childbirth, Rh+ red blood cells enter the mother's blood and stimulate the formation of D antibodies. D antibodies can cross the placenta in subsequent pregnancies. The reaction between the D antibody and the D antigen can destroy red blood cells and cause hemolysis in the newborn.

To prevent the Rh- mother from being sensitized by the Rh+ baby's D antigen, inject the mother just before the birth of her first child with a small amount of D antibody.

Passive agglutination reaction

Soluble antigens are adsorbed onto the surface of borrowed substrates such as bentonite beads, latex beads, but most commonly sheep erythrocytes. These particles agglutinate with antibodies in the presence of antigens that adhere to their surface. These particles are quite large so that the positive reaction is detectable with the naked eye. In the case of red blood cells used as antigen carriers, the reaction is referred to as passive erythrocyte agglutination.

To detect antigens, antibodies are attached to a borrowed platform. When an antibody encounters a specific antigen, agglutination occurs. This type is called a reverse passive agglutination reaction.

Passive agglutination is more sensitive than direct agglutination because of the relatively large size of the antigen carriers and is more specific than direct agglutination because antigens or antibodies can be purified. before mounting on the borrowed platform. This type of reaction is used in the diagnosis of many infections such as plague, Whitmore, meningitis ...

Reaction to prevent agglutination of red blood cells

There are viruses that are able to agglutinate the erythrocytes of certain animals and that response is inhibited by viral antisera. It is a reaction that prevents the agglutination of red blood cells. Hemocytosis is used to diagnose many viral diseases such as influenza, mumps, dengue, smallpox, etc.